Optical discs, which represent one type of information media, are currently available in two different varieties: optical recording discs which enable writing or rewriting of information, and read-only discs in which the information has been pre-recorded onto the disc.
A groove (guide channel) that is used for tracking and the like is formed in the disc substrate of an optical recording disc, and a recording layer comprising a phase change material or an organic dye material is laminated on top of the disc substrate. When the laser beam is irradiated onto the recording layer, the recording layer undergoes a chemical or physical change, thus forming a recording mark. In contrast, in the case of a read-only disc, recording marks (information pits) are formed in advance as part of an uneven pattern on the disc substrate. When a reading laser beam is irradiated onto these recording marks, the quantity of reflected light varies, and by detecting these variations, the information is able to be read (played back).
In order to manufacture a disc substrate with an uneven pattern of grooves, information pits, and the like, a stamper is used in which the negative pattern (which is itself a type of uneven pattern) of the desired uneven pattern has been formed. For example, a method of manufacturing a disc substrate by conducting injection molding using a mold with the above stamper secured inside the cavity, thereby transferring the negative pattern to the resin used to fill the cavity, is common.
A stamper with an uneven pattern is usually formed from a metal stamper containing Ni or the like. In the steps required for manufacturing this stamper, first a photoresist master with the negative pattern of the uneven pattern of the stamper is prepared, and a metal film is then formed on this photoresist master by plating. Subsequently, the metal film is separated from the photoresist master, and then subjected to a series of predetermined treatments such as surface washing to form the stamper.
As follows is a description of the manufacturing process for a photoresist master 1, with reference to the conventional photoresist master 1 shown in FIG. 7. First, a photoresist layer 4 is formed on top of a glass substrate 2. Next, the photoresist layer 4 is exposed using a patterning laser beam such as a laser, and the latent image pattern is developed. This enables the production of the photoresist master 1 with an uneven pattern 6 formed in the photoresist layer 4.
In order to use this photoresist master 1 to prepare a stamper 20 by plating, first, as shown in FIG. 8, a thin metal film 8 containing a Ni material or the like is formed on the surface of the uneven pattern 6 using a process such as electroless plating, thereby imparting conductivity to the photoresist master 1.
Subsequently, electroplating is conducted with the thin metal film 8 as a backing, thereby forming a metal film 10 containing Ni or the like. By removing the thin metal film 8 and the metal film 10 from the photoresist master 1, a stamper 20 containing the transferred uneven pattern 6 can be obtained.
In recent years, as the capacity of optical recording media has increased, uneven patterns such as grooves have become much finer, meaning errors in the pattern shape have a large effect on the recording and reading accuracy. Accordingly, it is desirable to form a sharp uneven pattern on the disc substrate, but in order to achieve this sharp pattern, the uneven pattern of the photoresist layer 4, which is the basis for the pattern, must be formed with a high level of precision (sharpness).
The minimum width of the latent image pattern formed on the photoresist layer 4 is limited by the spot diameter of the laser beam when it reaches the photoresist layer 4. When λ is the laser wavelength, and NA is the numerical aperture of the objective lens of the irradiating optical system, then the spot diameter w is represented by the formula w=k·λ/NA. k is a constant that is determined by the aperture shape of the objective lens and the intensity distribution of the incident light beam.
However, even in the case of patterns with widths that theoretically do not exceed the spot diameter limit, if the photoresist layer 4 is thin, then problems of inadequate sharpness can arise due to factors such as shallowness of the uneven pattern transferred to the stamper, or rounding of the shape of the uneven pattern (this is known as pattern sag). It is thought that these problems are caused by fluctuations occurring in the thickness of the photoresist layer 4 (this is known as film thinning) during typical exposure and developing operations. It is thought that these thickness fluctuations are caused by laser beam reflection between the photoresist layer 4 and the glass substrate 2, with this reflection causing excessive exposure of the photoresist layer 4.
The inventor of the present invention has clarified that forming a light absorption layer between the glass substrate 2 and the photoresist layer 4 is an effective way of resolving these problems. By so doing, the light absorption layer can absorb the laser beam and suppress any light reflection, and consequently a sharper exposure and development can be achieved than in a conventional process.
However, based on further research, the inventor of the present invention noticed that a photoresist master 1 with a light absorption layer displayed some problems relating to the formation of the thin metal film 8 by electroless plating. Specifically, it was surmised that a photoresist master 1 in which the light absorption layer was partially exposed was prone to increases in fine irregularities (fine defects) during the electroless plating process. In other words, it was discovered that even though the same method was used to form the thin metal film, on some occasions when the stamper was removed, for some reason or other fine irregularities (fine defects) had been formed on the surface of the uneven pattern of the stamper. During playback these fine irregularities manifest as noise, meaning that despite the attempt to improve the recording capacity by effectively utilizing a light absorption layer, in reality a decrease occurs in the recording and playback performance.
If this problem can be resolved, then the manufacture of a stamper with a sharp uneven pattern should be possible using a photoresist master with a light absorption layer. In other words, it became clear that a sharp uneven pattern that had been formed on the photoresist master through the effects of the light absorption layer, could be transferred faithfully to a stamper.